How Nuclear Reactors Work

When radioactive elements were discovered more than a century ago, scientists realized that radiation could be a powerful source of energy. But many advances in technology would be needed before they could harness nuclear energy to generate electricity effectively and safely.

Nuclear reactors are very sophisticated, but they use a very simple process.

Nuclear energy begins with uranium. It’s a heavy metal that is mined in Canada, and is mildly radioactive. You could stand safely next to uranium fuel for a long time.

Natural uranium has three varieties, or “isotopes,” with slightly different weights. One variety, uranium-235, can split – or “fission” – when hit with tiny particles called neutrons. When a uranium atom splits, it releases tremendous heat, which can be converted into electricity.

A chain reaction

Splitting uranium releases more neutrons, as well as heat. If they run into other uranium-235 atoms, the process starts over – another split, more heat, and more released neutrons.

This is called a “chain reaction.” Controlling that chain reaction is what nuclear reactors do.

The nuclear process

Turning that chain reaction into electricity takes several steps.

Operators load uranium fuel into a reactor core. Neutrons strike it, starting the chain reaction, which can continue as long as fissionable fuel remains in the core.

The neutrons in the chain reaction travel at many different speeds, but the slow ones work best in splitting uranium atoms. The core sits in a moderator – a fluid such as water – that slows the neutrons down.

Reactor operators keep the reaction controlled and steady, so that the reactor generates heat without getting too hot. They move control rods in and out of the reactor core. The rods are made of materials that absorb neutrons and slow or stop fission as needed.

The reactor could get very hot, so another fluid, a “coolant,” circulates through the reactor core to cool it down.

Like water poured on hard-boiled eggs to cool them, the coolant gets hot – but this is very useful heat. Despite its name, the coolant boils into high-pressured steam. The pressure pushes the steam to turn turbines.

All modern reactors use the same basic process, but some use different types of moderators, coolants, and grades of fuel. For the CANDU reactors used in Canada, the moderator is heavy water, the coolant is heavy water, and the fuel is natural uranium.

How efficient are nuclear reactors?

It might be surprising to learn that all these steps lead to the simple act of boiling water. But this process is very efficient. And because uranium contains so much energy, nuclear reactors make economic sense, too.

A single 20-gram uranium fuel pellet can produce the same amount of energy as 400 kilograms of coal, 410 litres of oil, or 350 cubic metres of natural gas. Eight pellets could power an average house for a full year.